Incorrect index mode in Get Data Streams API

[ElenaStoeva]

When we create an index template with a set index mode setting and a data stream that matches the same index pattern, after updating the index mode in the index template, the index mode is not updated in the data stream:

Screen.Recording.2025-03-25.at.11.01.33.mov

Related to #122486
Blocking elastic/kibana#215683

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[PeteGillinElastic]

Hi @ElenaStoeva . If I'm following the video correctly, this is working as intended.

Index templates are used to determine the settings of an index or data stream when it is created. If you modify a template, it won't change the settings of existing indices or data streams which were based on it, it'll only affect new ones which are created in the future. See the first paragraph in the docs.

Can you please confirm whether this explains the behaviour you're seeing?

(Aside: Things get a little more complicated for data stream backing indices, as changing e.g. the mappings in an index template will affect new backing indices of existing data streams as they roll over, but you're looking at a property of the data stream itself here. I'm not even sure that it's possible to change the index mode of an existing data stream, I think you may have to reindex, although I'm not 100% sure on that.)

[ElenaStoeva]

Hi @PeteGillinElastic, with the current behavior, the data stream's index mode updates only after a rollover is performed (see the video below).

recording.mov

The Kibana Management team requested the addition of index mode to the Data Streams API so we can correctly display the index mode in the Kibana data streams table. Based on my discussion with @dakrone, this field was intended to reflect the index mode set in the index template associated with the data stream. However, it currently does not—only updating after a rollover.

Example flow: a user wants to update the index mode of a data stream by modifying the index mode in the associated index template. After making this change, they navigate to the Data Streams page expecting to see the updated index mode, but instead, the old index mode is still displayed. This discrepancy can be confusing.

[PeteGillinElastic]

Thanks @ElenaStoeva , it sounds like there's been some misunderstanding somewhere... I've asked @dakrone to pitch in.